Non-foamed polyurethane is widely used in the manufacture of lacquers, adhesives, sealants, molded products, films, and coatings. In these applications, the appearance and the mechanical properties can be seriously affected by the presence of gas bubbles in the product. Substantially non-porous polyurethane plastics are generally prepared by reacting an organic polyisocyanate with an organic compound such as a polyol containing active hydrogen containing groups, polyamine compounds, or polyhydroxylated compounds. Polyols or polyhydroxylated compounds are known to absorb water and generally are the source for the introduction of water into the formulation. Moisture is introduced either in the polyhydroxylated compound or in some other ingredient, and this moisture can react with the organic polyisocyanate to produce urea linkages and carbon dioxide. The urea linkages are strong and usually desirable; however, the carbon dioxide causes bubbles to appear in the product. In many cases, the presence of bubbles in the product weakens the structure and may render the product unsatisfactory. In practice, the reaction is carried out at ambient temperature by simply mixing the two components, each containing one of the two reagents, at least one of the components being liquid at ambient temperature. In some applications, especially in producing elastomers, the reaction temperature may be as high as 100.degree. C. The mixture remains liquid or at least fluid during a certain period of time, which permits its use. This period or working time, which precedes the curing of the product, is commonly called the pot life.
Since it is known that the general appearance and the mechanical properties of polyurethane can be improved by incorporating mineral fillers which are inert with respect to the reagents and do not affect their polycondensation, the idea of using dehydrating agents and more particularly zeolites as the additional filler for the component containing the hydroxylated reagent was exploited. Other materials including pigments, catalysts, and solvents may also be incorporated into the polyurethane and are generally combined with the polyhydroxylated compound prior to the reaction.
This type of filler proves to be advantageous for any application of polyurethane in the form of thin films, paints, lacquers, floor and wall coverings because it neutralizes the diffusion of the ambient humidity into the polyurethane during polymerization. Differential curing of the upper layer, which is the source of cracking of the finished product, is thus avoided.
However, the zeolites which have been used for dehydration include the zeolites 4A, 5A, and 10A which possess the highly unfavorable property of adsorbing nitrogen from the air at ambient temperatures, and the desorption of this nitrogen caused by the exothermic nature of the polymerization reaction again creates undesirable microbubbles. U.S. Pat. No. 3,326,844, hereby incorporated by reference, discloses the use of such molecular sieve zeolites in polyurethane compositions and is relied upon for examples of polyols, polyamines, and other organic compounds containing active hydrogen containing groups which are reactive with an isocyanate group suitable for inclusion in polyurethane compositions.
U.S. Pat. No. 4,857,584 to Vandermeersch et al. discloses the problem that the above mentioned zeolites, in addition to adsorbing water, also tend to adsorb nitrogen from the air particularly when the polyhydroxylated component is stored after the addition of the zeolite. During the polymerization reaction, this nitrogen is released resulting in small bubbles in the finished product. Vandermeersch et al. propose to increase pot life by employing a cation exchanged form of zeolite 3A to avoid the problem of the nitrogen adsorption and to reduce the known reactivity of 3A zeolite in the polyhydroxylated compound (luring storage.
European Patent No. 0 346 604B1 discloses the use of a potassium zeolite 3A which has been treated with a strong acid to obtain a pH of between 7.5 to 9.5, as a dehydrating agent in polyurethane formulations. The pH of the acid treated potassium zeolite 3A is measured in an aqueous suspension according to DIN ISO 787/IX. The European disclosure indicated an increase in pot life when the pH of the zeolite 3A is reduced to a pH in the 7.5 to 9.5 range.
Currently, zeolite 3A is the most widely used zeolite for moisture scavenging in non-foamed polyurethane formulations. Zeolite 3A which has an alkaline surface is generally regarded as being reactive to polymerization reactions. Severe acid treatment of the zeolite 3A to significantly reduce the surface pH of zeolites may also change the crystal structure which would reduce the capacity of the zeolite to adsorb water. Methods are sought to produce a zeolitic composition which adsorbs a minimum of air and is essentially non-reactive in the polyhydroxylated compound as evidenced by a minimal impact on pot life of the polyurethane resin.